Adds Track-Only App Support (Addresses #119 and Sets Groundwork for #44) (#123)

- All Sources now have a "Track-Only" option that will prevent Obtainium from looking for APKs (though the App must still have a release of some kind so that a version string can be grabbed).
    - These Apps cannot be installed through Obtainium, but update notifications will still be sent.
    - The user needs to manually mark them as updated when appropriate.
    - This addresses issue #119.
    - It also partially addresses #44 by allowing some sources to be configured as "Track-Only"-only. The first such source (APKMirror) will be added later.
- Includes various UI changes to accommodate the above change.
- Also makes App loading a bit more responsive (sending Obtainium to the background then returning will now cause App re-load to pick up changes in App versioning that may have been made in the meantime, for instance through update checking).

lib/app_sources/fdroid.dart

@@ -48,9 +48,6 @@ class FDroid extends AppSource {
           .where((element) => element['versionName'] == latestVersion)
           .map((e) => '${apkUrlPrefix}_${e['versionCode']}.apk')
           .toList();
-      if (apkUrls.isEmpty) {
-        throw NoAPKError();
-      }
       return APKDetails(latestVersion, apkUrls);
     } else {
       throw NoReleasesError();

lib/app_sources/github.dart

@@ -11,9 +11,9 @@ class GitHub extends AppSource {
   GitHub() {
     host = 'github.com';
 
-    additionalDataDefaults = ['true', 'true', ''];
+    additionalSourceAppSpecificDefaults = ['true', 'true', ''];
 
-    moreSourceSettingsFormItems = [
+    additionalSourceSpecificSettingFormItems = [
       GeneratedFormItem(
           label: 'GitHub Personal Access Token (Increases Rate Limit)',
           id: 'github-creds',
@@ -51,7 +51,7 @@ class GitHub extends AppSource {
           ])
     ];
 
-    additionalDataFormItems = [
+    additionalSourceAppSpecificFormItems = [
       [
         GeneratedFormItem(label: 'Include prereleases', type: FormItemType.bool)
       ],
@@ -96,8 +96,8 @@ class GitHub extends AppSource {
   Future<String> getCredentialPrefixIfAny() async {
     SettingsProvider settingsProvider = SettingsProvider();
     await settingsProvider.initializeSettings();
-    String? creds =
-        settingsProvider.getSettingString(moreSourceSettingsFormItems[0].id);
+    String? creds = settingsProvider
+        .getSettingString(additionalSourceSpecificSettingFormItems[0].id);
     return creds != null && creds.isNotEmpty ? '$creds@' : '';
   }
 
@@ -155,14 +155,11 @@ class GitHub extends AppSource {
       if (targetRelease == null) {
         throw NoReleasesError();
       }
-      if ((targetRelease['apkUrls'] as List<String>).isEmpty) {
-        throw NoAPKError();
-      }
       String? version = targetRelease['tag_name'];
       if (version == null) {
         throw NoVersionError();
       }
-      return APKDetails(version, targetRelease['apkUrls']);
+      return APKDetails(version, targetRelease['apkUrls'] as List<String>);
     } else {
       rateLimitErrorCheck(res);
       throw getObtainiumHttpError(res);

lib/app_sources/gitlab.dart

@@ -33,7 +33,7 @@ class GitLab extends AppSource {
       var entry = parsedHtml.querySelector('entry');
       var entryContent =
           parse(parseFragment(entry?.querySelector('content')!.innerHtml).text);
-      var apkUrlList = [
+      var apkUrls = [
         ...getLinksFromParsedHTML(
             entryContent,
             RegExp(
@@ -48,9 +48,6 @@ class GitLab extends AppSource {
             .where((element) => Uri.parse(element).host != '')
             .toList()
       ];
-      if (apkUrlList.isEmpty) {
-        throw NoAPKError();
-      }
 
       var entryId = entry?.querySelector('id')?.innerHtml;
       var version =
@@ -58,7 +55,7 @@ class GitLab extends AppSource {
       if (version == null) {
         throw NoVersionError();
       }
-      return APKDetails(version, apkUrlList);
+      return APKDetails(version, apkUrls);
     } else {
       throw NoReleasesError();
     }

lib/app_sources/signal.dart

@@ -24,14 +24,12 @@ class Signal extends AppSource {
     if (res.statusCode == 200) {
       var json = jsonDecode(res.body);
       String? apkUrl = json['url'];
-      if (apkUrl == null) {
-        throw NoAPKError();
-      }
+      List<String> apkUrls = apkUrl == null ? [] : [apkUrl];
       String? version = json['versionName'];
       if (version == null) {
         throw NoVersionError();
       }
-      return APKDetails(version, [apkUrl]);
+      return APKDetails(version, apkUrls);
     } else {
       throw NoReleasesError();
     }

lib/app_sources/sourceforge.dart

@@ -49,9 +49,6 @@ class SourceForge extends AppSource {
           apkUrlListAllReleases // This can be used skipped for fallback support later
               .where((element) => getVersion(element) == version)
               .toList();
-      if (apkUrlList.isEmpty) {
-        throw NoAPKError();
-      }
       return APKDetails(version, apkUrlList);
     } else {
       throw NoReleasesError();